Method and system for removal of carbon dioxide from a process gas

ABSTRACT

Disclosed is a method of removing carbon dioxide from a process gas, the method comprising: contacting an ammoniated solution with the process gas in an absorption arrangement  101 , the ammoniated solution capturing at least a part of the carbon dioxide of the process gas, wherein the molar ratio, R, of ammonia to carbon dioxide in the ammoniated solution is controlled such that substantially no precipitation of solids occurs within the absorption arrangement  101 ; allowing ammoniated solution including captured carbon dioxide to exit the absorption arrangement  101 ; cooling the ammoniated solution that has exited the absorption arrangement, wherein at least a part of the captured carbon dioxide is precipitated as solid salt; separating at least a part of the precipitated salt from the ammoniated solution; heating the ammoniated solution from which the at least a part of the precipitated salt has been separated, such that substantially no solids are present in the heated ammoniated solution; and allowing the heated ammoniated solution to re-enter the absorption arrangement  101 . Disclosed is also a system for removal of carbon dioxide from a process gas.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 12/622,653, filed Nov. 20, 2009, and is a continuation-in-partof U.S. patent application Ser. No. 12/560,004, filed Sep. 15, 2009,both of which are incorporated by reference herein in their entirety.

TECHNICAL FIELD

The present invention relates to a method for removal of carbon dioxidefrom a process gas by contacting the process gas with an ammoniatedsolution.

BACKGROUND

Most of the energy used in the world today is derived from thecombustion of carbon and hydrogen containing fuels such as coal, oil andnatural gas, as well as other organic fuels. Such combustion generatesflue gases containing high levels of carbon dioxide. Due to concernsabout global warming, there is an increasing demand for the reduction ofemissions of carbon dioxide to the atmosphere, why methods have beendeveloped to remove the carbon dioxide from flue gases before the gas isreleased to the atmosphere.

WO 2006/022885 (U.S. patent application Ser. No. 11/632,537, filed Jan.16, 2007, and which is incorporated by reference herein in its entirety)discloses one such method of removing carbon dioxide from a flue gas,which method includes capturing carbon dioxide from the flue gas in aCO₂ absorber by means of an ammoniated solution or slurry. The CO₂ isabsorbed by the ammoniated solution in the absorber at a reducedtemperature of between 0° C. and 20° C., after which the ammoniatedsolution is regenerated in a regenerator under elevated pressure andtemperature to allow the CO₂ to escape the ammoniated solution asgaseous carbon dioxide of high purity.

SUMMARY

An objective of the present invention is to improve the method of carbondioxide absorption with an ammoniated solution.

This objective, as well as other objectives that will be clear from thefollowing discussion, is according to one aspect achieved by a method ofremoving carbon dioxide from a process gas, the method comprising:contacting an ammoniated solution with the process gas in an absorptionarrangement, the ammoniated solution capturing at least a part of thecarbon dioxide of the process gas, wherein the molar ratio, R, ofammonia to carbon dioxide in the ammoniated solution is controlled suchthat substantially no precipitation of solids occurs within theabsorption arrangement; allowing ammoniated solution including capturedcarbon dioxide to exit the absorption arrangement; cooling theammoniated solution that has exited the absorption arrangement, whereinat least a part of the captured carbon dioxide is precipitated as solidsalt; separating at least a part of the precipitated salt from theammoniated solution; heating the ammoniated solution from which the atleast a part of the precipitated salt has been separated, such thatsubstantially no solids are present in the heated ammoniated solution;and allowing the heated ammoniated solution to re-enter the absorptionarrangement.

The absorption arrangement may comprise one or several absorbers, suchas absorption stages. A plurality of absorbers of the absorptionarrangement may be arranged together in a common frame or casing, orarranged separate from each other only connected via piping, conduitsetc. In its simplest design, the absorption arrangement may compriseonly one absorber. This simple design will also simplify the carbondioxide removal method and will reduce the maintenance costs for thearrangement. The absorber or absorbers may be of any design that allowsdirect contact between the ammoniated solution and the process gas totake place within the absorber.

By contacting the ammoniated solution with the process gas, carbondioxide may be removed from the process gas and captured by theammoniated solution by crossing the formed interface between the processgas and the ammoniated solution.

There is a limit to how much carbon dioxide the ammoniated solution maycapture, i.e. when the ammoniated solution reaches saturation. Thislimit depends on e.g. the pressure and temperature of the solution. Bycooling the ammoniated solution, the ability of the solution to dissolvethe carbon dioxide is reduced, whereby at least a part of the capturedcarbon dioxide is precipitated as solid salt. Even if the ammoniatedsolution has not reached saturation in the absorption arrangement and nosolids have been precipitated prior to the cooling of the solution, thecooling of the ammoniated solution may allow for precipitation ofcaptured carbon dioxide in the form of a solid salt. Thus, at least partof the captured carbon dioxide may be separated from the ammoniatedsolution, e.g. by a separator, by removing at least a part of theprecipitated solids.

The ammoniated solution after separation may be saturated with carbondioxide since only the carbon dioxide in solid precipitated form isremoved, not carbon dioxide dissolved in the solution. By heating theammoniated solution, the ability of the solution to dissolve carbondioxide is increased even though the molar ratio R is unchanged,allowing the ammoniated solution to return to the absorption arrangementto capture more carbon dioxide without precipitation of solids.

By cooling the ammoniated solution, removing the solids, and re-heatingthe solution, most of the ammoniated solution may be returned to theabsorption arrangement to capture more carbon dioxide withoutprecipitation of solids. Thus, there is no need to regenerate the entiresolution stream. Instead, the much smaller volume of solids, andoptionally some solution, removed by separation and having a much highercarbon dioxide concentration may be transferred to a regenerator. Sincethe regenerator applies increased pressure and temperature to the solidmaterial, solution, suspension or slurry being regenerated in order toobtain leaving carbon dioxide of high purity, the energy consumption ismuch reduced if the volume of the solution, suspension or slurry isreduced and the carbon dioxide concentration is increased.

Also, by inducing precipitation of solids by cooling the ammoniatedsolution, carbon dioxide in the form of solid salt may be removed fromthe ammoniated solution even though the ammoniated solution exiting theabsorption arrangement contains no precipitated solids, i.e. theammoniated solution exiting the absorption arrangement might be rich incarbon dioxide but not completely saturated or supersaturated and stillallow for removal of carbon dioxide in solid form by e.g. a separator.This implies that the precipitation of solids within the absorptionarrangement and the absorber may be avoided compared with if no coolingwas performed.

Precipitation of solids in the absorption arrangement may be undesirablesince the solids may clog pipes, valves, pumps, absorbers etc., and mayalso increase the wear of the absorption arrangement due to increasedabrasion by the ammoniated solution flow. If there is no, precipitationin the absorption arrangement, the absorption arrangement may not haveto be designed to accommodate for solid particles in the ammoniatedsolution whereby the absorption arrangement may be designed in a simplerand cheaper way and for more efficient carbon dioxide capture, e.g. by amore effective packing material in the absorber if a packing material isused, which packing material might otherwise be clogged and result inexcessive pressure drop. Also, the maintenance of the absorptionarrangement may be greatly reduced.

The amount of captured carbon dioxide in relation to the amount ofammonia in the ammoniated solution is illustrated with the molar ratio Rbetween the ammonia (NH₃) and the carbon dioxide (CO₂) present in theammoniated solution, i.e. R═[NH₃]/[CO₂]. According to the presentmethod, R is kept at a level such that substantially no precipitationoccurs within the absorption arrangement.

Controlling the R of the ammoniated solution such that no precipitationof solid salt occurs within the absorption arrangement may be achievedin many different ways, such as by controlling the flow rate of theammoniated solution exiting the absorption arrangement and thus alsocontrolling the flow rate of ammoniated solution re-entering theabsorption arrangement, by controlling the temperature to which theammoniated solution is cooled down in order to induce precipitation aswell as controlling the temperature to which the ammoniated solution isheated before re-entering the absorption arrangement, by controlling aflow of ammoniated solution having an R value above the precipitationthreshold into the absorption arrangement other than the flow ofre-entering separated ammoniated solution and/or by controlling thetemperature(s) of the absorption arrangement and its different parts.

The molar ratio, R, of ammonia to carbon dioxide in the absorptionarrangement is kept at a level such that substantially no precipitationoccurs within the absorption arrangement at the temperature and pressureof the ammoniated solution in the absorption arrangement. This impliesthat the molar ratio R of the ammoniated solution that exits theabsorption arrangement is also high enough to avoid precipitation. Thus,R of the solution that exits the absorption arrangement may be at least1.8, more preferably at least 1.9, such as about 1.95, to avoidprecipitation at the operating temperature of the absorptionarrangement.

The temperature of the ammoniated solution that exits the absorptionarrangement may be between 10° C. and 25° C., such as between 15° C. and20° C., at which temperature range the ammoniated solution is saturatedat an R of about 1.95. It may be undesirable to have a lower temperaturesince then less carbon dioxide may be captured before the solutionreaches saturation and solid salt is precipitated. In other words, the Rof saturation will be higher. It may also be undesirable to have ahigher temperature since too much ammonia may then evaporate from theammoniated solution, lowering the R of the solution and reducing theamount of carbon dioxide that may be captured by the ammoniated solutionbefore saturation and precipitation, as well as contaminating theprocess gas.

It may be advantageous to operate the absorption arrangement with an Rof the exiting ammoniated solution that is close to saturation, such asan R of less than 4.0, conveniently less than 2.5, more preferably lessthan 2.0, such as 1.95. This implies that the ammoniated solution may beused to, or close to, its full potential, capturing as much carbondioxide as possible without any precipitation, making the carbon dioxideremoval method more efficient.

After heating the ammoniated solution, after separation of solids, theammoniated solution is re-introduced to the absorption arrangement. Theammoniated solution that re-enters the absorption arrangement may havean R value similar to the R value of the ammoniated solution that exitsthe absorption arrangement, since both ammonium and captured carbondioxide have been removed from the ammoniated solution due toprecipitation and separation. Thus, the ammoniated solution thatre-enters the absorption arrangement may have an R of at least 1.8,conveniently at least 1.9, such as at least 1.95. In analogy, theammoniated solution re-entering the absorption arrangement may have an Rof less than 4.0, conveniently less than 2.5, more preferably less than2.0, such as 1.95. However, if mainly ammonium bicarbonate isprecipitated, the R value of the re-entering ammoniated solution may behigher than the R value of the exiting solution, such as an R value ofat least 2.0, conveniently at least 2.2 such as at least 2.5.

The temperature to which the ammoniated solution which has exited theabsorption arrangement is cooled may conveniently be between 0° C. and7° C., such as between 2° C. and 5° C.

The capturing of carbon dioxide by the ammoniated solution may beexothermic, why the solution may be heated during its capturing ofcarbon dioxide. It may thus be convenient for the ammoniated solutionthat re-enters the absorption arrangement to have a lower temperaturethan the ammoniated solution that exits the absorption arrangement. Itmay also be convenient to keep down the temperature of the ammoniatedsolution to avoid evaporation of the ammonia into gaseous phase. Ascarbon dioxide is captured, the temperature of the solution is increasedwhereby the capacity of the solution to capture carbon dioxide withoutprecipitation is also increased. It may thus be convenient to controlthe temperature of the ammoniated solution re-entering the absorptionarrangement such that all of it, or at least a part or fraction of it,has a temperature of between 0° C. and 10° C., such as of between 3° C.and 7° C.

The R of the ammoniated solution in the absorption arrangement may atleast partly be controlled by introducing a controlled amount ofammoniated solution having an R which is higher than the R of theammoniated solution that exits the absorption arrangement, such asbetween 2.2 and 5.0, apart from the ammoniated solution that re-entersthe absorption arrangement. This ammoniated solution may e.g. be carbondioxide lean ammoniated solution from a regenerator or be freshammoniated solution that has not been recycled.

According to another aspect of the present disclosures, there isprovided a system for removal of carbon dioxide from a process gas, thesystem comprising: an absorption arrangement arranged to allow contactbetween the process gas and an ammoniated solution within the absorptionarrangement such that at least a part of the carbon dioxide of theprocess gas is captured by the ammoniated solution, and the absorptionarrangement being arranged to, with regard to the ammoniated solution,only accommodate ammoniated solution without solids; a first heatexchanger arranged to cool the ammoniated solution including capturedcarbon dioxide after it has exited the absorption arrangement; aseparator arranged to remove at least a part of any solids in the cooledammoniated solution; a second heat exchanger arranged to heat theammoniated solution after it has exited the separator; and piping and/orconduits connecting, and arranged to allow a flow of the ammoniatedsolution between, the absorption arrangement and the first heatexchanger, the first heat exchanger and the separator, the separator andthe second heat exchanger, as well as the second heat exchanger and theabsorption arrangement.

It may be convenient to use the system for removal of carbon dioxide inperforming the method discussed above.

It may be convenient to arrange the first and second heat exchangers tocooperate with each other such that the ammoniated solution being cooledin the first heat exchanger is at least partly cooled by the ammoniatedsolution being heated in the second heat exchanger as cooling medium,and the ammoniated solution being heated in the second heat exchanger isat least partly heated by the ammoniated solution being cooled in thefirst heat exchanger as heating medium. This may lead to a reduction ofthe energy needed to run the system.

The system for removal of carbon dioxide from a process gas may furthercomprise a control system configured to control the NH₃-to-CO₂ moleratio (R) of the ammoniated solution such that substantially noprecipitation of solids occurs within the absorption arrangement whenthe absorption arrangement is in use.

The discussion above relating to the method is in applicable parts alsorelevant to the system. Reference is made to that discussion.

The absorption arrangement of the system may in one embodiment comprise:a first absorption stage arranged to receive the process gas and contactit with a first part of the ammoniated solution; a second absorptionstage arranged to receive process gas which has passed the firstabsorption stage and contact it with a second part of the ammoniatedsolution; a first sump vessel; and a second sump vessel; wherein saidfirst absorption stage comprises a liquid collection receptacle arrangedto collect ammoniated solution from the first absorption stage anddeliver it to the first sump vessel, and said second absorption stagecomprises a liquid collection receptacle arranged to collect ammoniatedsolution from the second absorption stage and deliver it to the secondsump vessel.

A multi-stage absorption arrangement, in which a number of differentabsorption stages, i.e. absorbers, operate under different conditionsbut arranged in the same frame or casing, may often constitute asuperior alternative to multiple single-stage absorbers arranged inseries. Advantages of the multi-stage absorption arrangement include,e.g., lower capital costs for vessels, packing and foundations.

This embodiment is based on the insight that the efficiency andversatility of a multi-stage absorption arrangements may besignificantly improved by division of the sump of the absorptionarrangement into two or more separate sections, referred to herein assump vessels. Each of the sump vessels is arranged to receive usedammoniated solution from one or more predetermined absorption stages.The use of multiple sump vessels facilitates recycling of usedammoniated solution within the absorption arrangement since ammoniatedsolution from one or more absorption stages having similar compositionand properties may be collected in a first sump vessel, while ammoniatedsolution from one or more other absorption stages having similarcomposition and properties, different to the composition and propertiesof the ammoniated solution collected in the first sump vessel, may becollected in a second sump vessel. The ammoniated solution collected inthe first and second sump vessels may be recycled, possibly afteradjustment of the composition and properties of the respective solutionto a desired absorption stage. Thus, the use of multiple sump vesselsallows the operating conditions, such as for example temperature,ammoniated solution composition and flow rate, of each absorption stageto be varied within a wide range.

The system may further comprise a control system configured to maintainthe mole ratio R of the ammoniated solution in the first sump vesselwithin a range of 1.8 to 2.5.

The system may further comprise a control system configured to maintainthe mole ratio R of the ammoniated solution in the second sump vesselwithin a range of 2.0 to 4.5.

The system may further comprise a control system configured to maintainthe temperature of the first sump vessel within a range of 10 to 25° C.,conveniently within a range of 15-20° C.

The system may further comprise a control system configured to maintainthe temperature of the second sump vessel within a range of 10 to 25°C., conveniently within a range of 15-20° C.

The system may comprise a single control system configured to maintainthe temperature and/or R value of the first and/or the second sumpvessels, or the system may comprise separate control systems formaintaining temperatures and R values, or for maintaining the first andthe second sump vessels.

In an embodiment, the control system comprises a device configured tointroduce NH₃ or a medium having an R higher than the R of theammoniated solution in at least one of the sump vessels into theammoniated solution of that sump vessel.

The above described and other features are exemplified by the followingfigures and detailed description.

BRIEF DESCRIPTION OF THE DRAWING

Referring now to the FIGURE, which is an exemplary embodiment:

FIG. 1 is a diagram generally depicting an embodiment of a CO₂ capturesystem that includes a multi-stage absorbing arrangement with two sumpvessels.

DETAILED DESCRIPTION

The process gas may be any type of process gas containing carbondioxide, such as process gas from any combustion device such asfurnaces, process heaters, incinerators, package boilers, and powerplant boilers.

The ammoniated solution may be any type of solution containing ammonia,such as a liquid solution, especially an aqueous solution. The ammoniain the ammoniated solution may be in the form of ammonium ions and/ordissolved molecular ammonia.

The capturing of CO₂ from the process gas by the ammoniated solution maybe achieved by the ammoniated solution absorbing or dissolving the CO₂in any form, such as in the form of dissolved molecular CO₂, carbonateor bicarbonate.

The solids formed in the ammoniated solution may mainly be ammoniumcarbonate and ammonium bicarbonate, especially ammonium bicarbonate.

The carbon dioxide removal system comprises piping and/or other conduitsthat connects the different parts of the system and is arranged to allowammoniated solution and process gas, respectively, to flow through thesystem as needed. The piping may comprise valves, pumps, nozzles etc. asappropriate to control the flow of ammoniated solution and process gas,respectively.

The one or several absorbers of the absorbing arrangement may have anydesign that allows the ammoniated solution to contact the process gas.It may be convenient with an absorber design in the form of a column,where the ammoniated solution flows from the top of the column to thebottom of the column and the process gas flows from the bottom of thecolumn to the top of the column, thus the solution and the gas may meetand mix with each other in the column, creating an interface between thesolution and the gas across which interface carbon dioxide may travelfrom the gas to the solution. The gas/solution contact may be increased,i.e. the interface area may be increased, by using a packing in thecolumn, thereby improving the carbon dioxide capturing. The respectiveflows of the process gas and the ammoniated solution within, as well asto and from, the absorption arrangement may be controlled by at leastone pumping system and/or by act of gravity.

If an absorber in the form of a column is used, the process gas mayenter the column via a pipe connected to the lower part of the column,travel upwards through the column and exit the column via a pipeconnected to the upper part of the column, and the ammoniated solutionmay enter via a pipe connected to the upper part of the column, traveldownwards through the column by action of gravity and exit the columnvia a pipe connected to the lower part of the column. The ammoniatedsolution and/or the process gas may additionally be recirculated in thecolumn. If the ammoniated solution is recirculated, the ammoniatedsolution may alternatively be entered into the column at the lower partof the column instead of at the upper part of the column, allowing arecirculation loop to transport the solution to the upper part of thecolumn. The column may be associated with a pumping system to effect therecirculation.

In order to control the temperature of the column, at least one heatexchanger may be associated with the column. The heat exchanger may e.g.form part of a recirculation loop for the ammoniated solution. Since thecapturing of carbon dioxide by the ammoniated solution is an exothermicreaction, the heat exchanger may be used to cool down the ammoniatedsolution to keep the interior of the absorber at a desired andsubstantially constant temperature.

Depending of the design of and the demands put on the absorptionarrangement, it may be convenient to use a plurality of absorbers inorder to remove a desired amount of the carbon dioxide from the processgas.

If a plurality of absorbers are used, they may have the same ordifferent designs. The absorbers may be serially connected to each otherto allow process gas and/or ammoniated solution to serially flow fromone absorber to another absorber. However, it should be noted that thegas and the solution may flow in different directions between theserially connected absorbers. If e.g. an absorption arrangementcomprises three serially connected absorbers, denoted x, y and z, thegas flow may be from absorber x to absorber y to absorber z, whereas theflow of the ammoniated solution may e.g. be from absorber y to absorberx to absorber z or in any other order.

The cooling and/or the heating, respectively, of the ammoniated solutionmay e.g. be done with heat exchangers, but any other means of heatingand/or cooling a liquid flow may alternatively or additionally be used.It has been realized that it might be advantageous to at least partlyperform the cooling and the heating by means of the same heatexchanger(s), in which heat exchanger the ammoniated solution exitingthe absorption arrangement is the heating medium and the ammoniatedsolution from which precipitated salt has been separated is the coolingmedium. Thus, energy may be conserved. Using the cooled and separatedammoniated solution as a cooling medium for cooling the ammoniatedsolution which has exited the absorption arrangement might not besufficient for cooling the ammoniated solution which has exited theabsorption arrangement, why it might be convenient to additionally use aregular cooling medium, such as cold water. The regular cooling mediummay be connected to the same heat exchanger as the separated ammoniumsolution, or to a separate heat exchanger. Thus, the ammoniated solutionexiting the absorption arrangement may be first cooled by the ammoniumsolution from the separator and then be additionally cooled by means ofthe regular cooling medium, or vice versa. Alternatively, the ammoniatedsolution is not used as a cooling or heating medium, but regular coolingand heating mediums are used instead.

The separation may be achieved by any means for separating particulatesolids from a liquid, but it may be convenient to use a separator. Sucha separator may be any type of separator able to separate, and thusremove, solid particles or material from the ammoniated solution.Depending on the requirements put on the separator, it might beconvenient to use a separator in the form of a hydrocyclone. Ahydrocyclone may be an efficient way of removing solids from theammoniated solution. The suspension or slurry of the ammoniated solutioncomprising solids enters the hydrocyclone where the suspension or slurryis separated into an overhead solution reduced in, or free from, solidsand an underflow rich in solids. It has been found that it may beconvenient with a solids content of the ammoniated solution comprisingsolids entering the hydrocyclone of between 5% and 10% by weight of theammoniated solution comprising solids entering the hydrocyclone.Ideally, substantially all the solids are removed from the ammoniatedsolution, giving an overhead solution substantially free from solids. Ithas been found that it may be convenient with a solids content of theoverhead solution of between 0% and 1% by weight of the overheadsolution. The underflow may be allowed to also contain some liquidsolution in order to facilitate transporting the solids in a liquidstream, thus some of the ammoniated solution may also be separated tothe underflow. The amount of liquid in the underflow may be enough totransport the solids in a liquid stream but without reducing the carbondioxide concentration more than necessary to allow this transportation.The underflow may be a leaving suspension or slurry, leaving theammoniated solution.

Regardless of the type of separator used, it may be convenient that mostor substantially all of the solids are removed from the ammoniatedsolution to a leaving suspension or slurry, in which suspension orslurry the amount of liquid has been balanced to allow transportation ofthe solids in a liquid stream but without reducing the carbon dioxideconcentration more than necessary to allow this transportation. It maybe convenient to have a solids content of at least 10% by weight of theleaving suspension or slurry, such as between 10% and 20% by weight ofthe leaving suspension or slurry.

With reference to FIG. 1, an embodiment in accordance with the presentdisclosure will now be described.

In this embodiment, a CO₂ capture system is provided that includes three(3) absorption stages, i.e. three absorbers. It is, however, possible toinclude more or fewer absorption stages in the capture system or toarrange them differently in relation to each other.

Referring to FIG. 1 an absorption arrangement 101 in the form of asingle absorption vessel is provided. The absorption arrangement 101 isconfigured to receive a process gas stream via an inlet 102 located nearthe bottom of the vessel 101 and to allow the process gas stream to passupward and through the absorption arrangement 101 to exit via an outlet103 located near the top of the vessel 101.

The process gas stream entering the absorption arrangement 101 willtypically contain less than one percent moisture and low concentrationsof SO₂, SO₃, HCl, and particulate matter which will typically be removedvia air pollution control systems (not shown) upstream from the CO₂capture system.

The absorption arrangement 101 is configured to absorb CO₂ that may becontained in a process gas stream, using an ammoniated solution. In anembodiment, the ammoniated solution may be composed of, for example,water and ammonium ions, bicarbonate ions, carbonate ions, and/orcarbamate ions.

The CO₂ capture system comprises three absorption stages 104, 105 and106, the first and third absorption stages 104 and 106 being connectedto a first sump vessel 107, and the second absorption stage 105 beingconnected to a second sump vessel 108 in a manner described in detailherein below.

The CO₂ capture system comprises two separate ammoniated solution sumpvessels 107 and 108, referred to herein as the first (107) and second(108) sump vessel. The term “separate” generally means that theammoniated solution in the first sump vessel 107 is not in continuousliquid contact with the ammoniated solution in the second sump vessel108. Although the first and second sump vessels are not in continuousliquid contact, the system may further comprise a conduit 109 fortransferring ammoniated solution from the second sump vessel 108 to thefirst sump vessel 107.

The first sump vessel 107 is arranged to receive used ammoniatedsolution from the first absorption stage 104 via liquid collectionreceptacle 110, and from the third absorption stage 106 via liquidcollection receptacle 112. The second sump vessel 108 is arranged toreceive used ammoniated solution from the second absorption stage 105via liquid collection receptacle 111. The first sump vessel is arrangedto supply ammoniated solution to the first absorption stage via asolution delivery path 113 and a liquid distribution device 114 and tothe third absorption stage via a solution delivery path 117 and a liquiddistribution device 118. The second sump vessel is arranged to supplyammoniated solution to the second absorption stage via a solutiondelivery path 115 and a liquid distribution device 116. The first and/orsecond sump vessels 107 and 108 are further configured for receiving CO₂lean ammoniated solution from a regenerator (not shown) and/or make-upNH₃.

In the embodiment shown in FIG. 1, the first and second sump vessels 107and 108 are formed by two sub-sections of the bottom portion 119 of theabsorption arrangement, below the first absorption stage 104.

The CO₂ capture system may further comprise a control system forcontrolling the NH₃-to-CO₂ mole ratio (R) in the first and second sumpvessel to be within a desired range. The control system may comprisesensors for automated or manual measurement of relevant parameters, suchas e.g. pH value, ammonia concentration and/or CO₂ concentration, anddevices, such as liquid connections, valves and pumps, configured foradjustment of such parameters, e.g. by addition of make-up NH₃ and/orremoval of CO₂. The system may comprise an automatic controller 134, bywhich the NH₃-to-CO₂ mole ratio is maintained at desired values in thefirst and second sump vessels 107 and 108. For example, the automaticcontroller 134 may be a general-purpose computer, application specificcomputing device or other programmable controller that receives inputsignals indicative of the R value from sensors 135 and 136 in the firstand second sump vessels 107 and 108. The automatic controller 134 mayprovide control signals to a pump 137, control valve, or other fluidflow adjusting device, to maintain R within the first sump vessel 107 towithin the desired range, and may provide control signals to the NH₃make-up supply and/or the lean solution supply from the regenerator tomaintain R within the desired range in the second sump vessel 108. In anembodiment, the R value in the first sump vessel is maintained in arange of 1.8 to 2.5, such as about 2.0, by replacing a portion of theammoniated solution in the first sump vessel 107 with higher Rammoniated solution from the second sump vessel 108 via conduit 109, andthe R value in the second sump vessel may be maintained in a range of2.0 to 4.0, such as about 2.5, by replacing the portion of ammoniatedsolution sent to the first sump vessel with CO₂ lean ammoniated solutionfrom the regenerator and/or make-up NH₃.

Each absorption stage 104, 105 and 106 is configured to include one ormore suitable gas-liquid mass transfer devices (MTD) 120, 121 and 122,respectively, a liquid distribution device 114, 116 and 118,respectively, and a solution delivery path (SDP) 113, 115 and 117,respectively.

Each mass transfer device 120, 121 and 122 is configured to contactammoniated solution with the process gas stream as the process gas flowsupwards through the absorption arrangement 101, counter current to theammoniated solution containing, for example, a dissolved mix of ammoniumions, carbonate ions, ammonium bicarbonate and/or carbamate ions. Masstransfer devices (MTD) 120, 121 and 122 may be, for example, structuredor random packing materials.

Liquid distribution device(s) 114, 116 and 118 are configured tointroduce ammoniated solution into the absorption arrangement 101. Eachliquid distribution device may be configured as, for example, one ormore spray head nozzles and/or conduit with perforations, holes and/orslots or a combination thereof.

Each SDP 113, 115 and 117 is configured to deliver a flow of ammoniatedsolution to the respective absorption stage via a liquid distributiondevice 114, 116 and 118, respectively. Each SDP will preferably includeone or more cooling systems, such as, for example, a heat exchangedevice 124, 126 and 127, for cooling ammoniated solution pumped throughthe SDP. A control system may also be provided for controlling the flowof the ammoniated solution and maintaining ammoniated solutiontemperature at a predetermined level or within a predeterminedtemperature range. The control system may include a controller, forexample a general purpose computer, an application specific computingdevice or other programmable controller, that receives input signalsfrom one or more temperature sensor and provides control signals to aheat exchange device to effect cooling or heating of the ammoniatedsolution. The control system may be integrated with the control systemdescribed above for controlling the R-value of the ammoniated solution,and the controller, e.g. computing device, may be the same. Withreference to FIG. 1, the first absorption stage 104 includes an SDP 113that is composed of conduit/pipe that connects the first sump vessel 107with liquid distribution device 114 via pump 123 and heat exchanger 124.The second absorption stage 105 includes an SDP 115 that is composed ofconduit/pipe that connects a second sump vessel 108 to the liquiddistribution device 116 via pump 125 and heat exchanger 126. The thirdabsorption stage 106 includes an SDP 117 that is composed ofconduit/pipe that connects the first sump vessel 107, with liquiddistribution device 118 via pump 123, heat exchanger 124 and heatexchanger 127.

Each absorption stage 104, 105 and 106 may comprise a device forcollecting ammoniated solution which has passed through the respectiveMTD 120, 121 and 122. Each such liquid collection receptacle 110, 111and 112 may be configured to collect all or a portion of the liquidwhich passes through respective MTD. Each liquid collection receptaclemay for example be configured to collect substantially all, i.e. about95% or more, such as 98% or more of the ammoniated solution which passesthrough respective MTD. Alternatively, a major portion of the ammoniatedsolution which passes through respective MTD may be collected, forexample more than 50%, such as more than 70% or more than 90% of theammoniated solution. The liquid collection receptacles may be arrangedor configured such that process gas rising up through the absorptionarrangement 101 may pass through or alongside the liquid collectionreceptacles. The liquid collection receptacles may for example comprisea sloped collection tray or bubble cap tray. The liquid collectionreceptacles may further comprise one or more liquid outlets configuredfor removal of liquid collected by the liquid collection receptacles.The liquid collection receptacle 110 of the first absorption stage isconnected to the first sump vessel 107 via conduit 129 which allows usedammoniated solution collected by the liquid collection receptacle 110 tobe directed to the first sump vessel 107 for recycling. The liquidcollection receptacle 111 of the second absorption stage is connected tothe second sump vessel 108 via conduit 130 which allows used ammoniatedsolution collected by the liquid collection receptacle 111 to bedirected to the second sump vessel 108 for recycling. The liquidcollection receptacle 112 of the third absorption stage is connected tothe first sump vessel 107 via conduit 131 which allows used ammoniatedsolution collected by the liquid collection receptacle 112 to bedirected to the first sump vessel 107 for recycling.

The liquid collection receptacles may further comprise a respectiveflush system (not shown) for cleaning. In some embodiments, liquid whichhas passed through the MTD of the first absorption stage 104 may becollected directly in a bottom portion of the absorption arrangement. Insuch embodiments, no further liquid collection receptacle may berequired for the first absorption stage 104.

The first absorption stage 104 is configured to contact a relatively lowR ammoniated solution received from the first sump vessel 107 via SDP113 with the process gas stream. This ammoniated solution is pumped fromthe first sump vessel 107 via pump 123 to the liquid distribution device114, which sprays the ammoniated solution downward and onto the masstransfer device 120. In this way the process gas stream comes intocontact with the ammoniated solution sprayed from liquid distributiondevice 114. The temperature of the ammoniated solution at absorptionstage 104 may be controlled to be in a range from 5° C. to 20° C. orhigher. After the ammoniated solution has been contacted with theprocess gas stream it is more rich in CO₂ (rich solution). This rich inCO₂ solution is discharged from absorption stage 104 to the first sumpvessel 107 via conduit 129. A portion of the ammoniated solution in thefirst sump vessel 107 may be pumped to a regenerator system (not shown)to increase the ammonia-to-CO₂ mole ratio (R) of the liquid.

The first absorption stage 104 may be configured to capture 50-80% ofthe carbon dioxide contained in the process gas entering the absorptionarrangement 101, conveniently 60-70%.

The second absorption stage 105 may be configured to capture 10-40% ofthe carbon dioxide contained in the process gas entering the absorptionarrangement 101, conveniently 20-30%. Here, relatively high R ammoniatedsolution from the second sump vessel 108 is sprayed via liquiddistribution device 116 onto the MTD 121. The high R solution sprayedvia the spray system 116 is contacted with the process gas stream as itflows from the first absorption stage 104 upward through the MTD 121 ofthe second absorption stage.

The absorption arrangement 101 may optionally further comprise a thirdabsorption stage 106 for further removal of CO₂ from the process gas andfor reduction of ammonia slip, i.e. evaporation of ammonia, from theprevious absorption stages.

The process gas rising upward in the absorption vessel 101 from thesecond absorption stage 105 contains a low concentration of CO₂ (forexample 20% or less, or 10% or less, of the CO₂ in the process gasentering the absorption arrangement 101) and a relatively highconcentration of NH₃ (for example from 5000 ppm up to 30000 ppm). Thehigh concentration of ammonia in the process gas (ammonia slip) from thesecond absorption stage 105 is a result of the high R of the ammoniatedsolution in the second absorption stage 105. A large portion of theammonia that has evaporated in the second absorption stage 105 may bere-captured back into the ammoniated solution via a third absorptionstage 106, which may operate at a lower temperature.

In the third absorption stage 106, a relatively small flow of ammoniatedsolution having a low temperature (for example less than 10° C. andconveniently about 5° C.) is sprayed via liquid distribution device 118onto the MTD 122 wherein it is contacted with the process gas stream asit flows upward through the MTD 122. The ammoniated solution dischargedfrom the third absorption stage 106 may be collected in the first sumpvessel 107 via conduit 131.

The absorption arrangement 101 is configured to provide for circulation,by means of a pump 140, of ammoniated solution collected at the bottomof the first sump vessel 107 to a combined cooling/heating heatexchanger 138 arranged to cool the ammoniated solution using theseparated ammoniated solution as a cooling medium. The combined heatexchanger 138 is connected to a cooling heat exchanger 139 to allowammoniated solution to flow from the combined heat exchanger 138 to thecooling heat exchanger 139. The cooling heat exchanger 139 is arrangedto further cool the ammoniated solution using cold water from the coldwater source 141 as a cooling medium. The cooling heat exchanger 139 isconnected to a separator in the form of a hydrocyclone 133 arranged toseparate solid material such as precipitated salt from the cooledammoniated solution flowing from the cooling heat exchanger 139 to thehydrocyclone 133. The hydrocyclone 133 is connected to a solidcollection tank 132 arranged to receive the solids rich underflow fromthe hydrocyclone 133. The hydrocyclone 133 is also connected to thecombined heat exchanger 138 which is arranged to heat the overflow lowon solids from the hydrocyclone 133 using the ammoniated solution fromthe first sump vessel 107 as a heating medium. The combined heatexchanger is connected to the first sump vessel 107 of the absorptionarrangement 101 to allow re-entry of the heated ammoniated solution.

An example of a method for removal of carbon dioxide from a process gasby means of the system of FIG. 1 may be summarized in the followingsteps:

In step 1, the ammoniated solution in the form of an aqueous solution,as well as the process gas, enters the absorption arrangement via pipes.The absorption arrangement may comprise one or a plurality of absorbersor absorption stages, conveniently in the form of packed columns orbeds.

In step 2, the ammoniated solution, as well as the process gas, entersthe first absorption stage. The ammoniated solution enters the firstabsorption stage at the top of the bed, after which the ammoniatedsolution flows downward though the MTD of the first absorption stage.Simultaneously, the process gas enters the first absorption stage at thebottom of the bed, after which the process gas flows upward though theMTD of the first absorption stage. The ammoniated solution and theprocess gas thus meet and are contacted with each other as they flowcounter currently in the first absorption stage. The packing of the bedof the MTD acts to increase the mixing and the contact area, interface,between the liquid phase and the gas phase in the bed. Carbon dioxide ofthe process gas travels from the gas phase into the liquid phase and isthus captured by the ammoniated solution. The ammoniated solution and/orthe process gas may be recirculated in the absorption arrangement.During this re-circulation, possibly outside of the absorptionarrangement, the temperature of the ammoniated solution may also beadjusted by means of a heat exchanger.

It should be noted that the ammoniated solution and/or the process gasmay have already passed though one or several absorbers or absorptionstages after entering the absorption arrangement prior to entering saidfirst absorption stage, depending on the design of the system.

In step 3, the ammoniated solution is contacted with the process gas inthe second absorption stage. The discussion above relating to the firstabsorption stage in step 2 is also relevant to the second absorptionstage in step 3.

In step 4, the ammoniated solution is contacted with the process gas inthe third absorption stage. The discussion above relating to the firstabsorption stage in step 2 is also relevant to the third absorptionstage in step 4.

In step 5, the ammoniated solution leaves the absorption arrangement viaa pipe or other conduit. The ammoniated solution leaving the absorptionarrangement may be taken from any part of the absorption arrangement,such as from the first or second sump vessels or from any one of theabsorption stages, e.g. from any one of the liquid collectionreceptacles of the absorption stages, or from several of these parts.

In step 6, the ammoniated solution enters at least one heat exchangerand is cooled down. As a result of the cooling, a part of the capturedcarbon dioxide is precipitated as salt. It may be preferred to use twoseparate heat exchangers, the first using cooled ammoniated solution ascooling medium and the second using cold water as cooling medium.

In step 7, the cooled ammoniated solution including salt solids enters ahydrocyclone, or other separating means. In the hydrocyclone, theammoniated solution is separated into a solid rich underflow and anoverhead solution with less than 1 wt % solids. Thus, most of the solidshave been removed from the ammoniated solution by the hydrocyclone. Thesolid rich underflow may be transferred to a solid collection tank ordirectly to a regenerator where it is subjected to increased temperatureand increased pressure in order to remove the captured carbon dioxide inthe form of a leaving carbon dioxide gas stream of high purity. The thusregenerated ammoniated solution from the underflow may then be allowedto re-enter the absorption arrangement to capture more carbon dioxide.

In step 8, the ammoniated solution, i.e. the overhead solution from thehydrocyclone, is reheated. In order to save energy, the reheating mayconveniently by made by means of the same first heat exchanger asdiscussed under step 6, with the ammoniated solution cooled in step 6 asheating medium. If needed, an additional heat exchanger with atraditional heating medium, such as warm water, may also be employed. Inheating the ammoniated solution, the solution is rendered unsaturatedwith respect to carbon dioxide, allowing it to capture more carbondioxide without inducing any precipitation.

In step 9, the reheated ammonium solution re-enters the absorptionarrangement to capture more carbon dioxide from the process gas. Theammoniated solution may re-enter the absorption arrangement at the topof any one or several of the absorption stages, or in any one or both ofthe sump vessels, or anywhere else in the absorption arrangement.

It should be noted that the method may be continuous. Thus all the stepsabove may occur concurrently involving different parts of the ammoniatedsolution.

While the invention has been described with reference to variousexemplary embodiments, it will be understood by those skilled in the artthat various changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims. Moreover, the use of the terms first, second, etc. do not denoteany order or importance or chronology, but rather the terms first,second, etc. are used to distinguish one element from another.

1. A method of removing carbon dioxide from a process gas, the methodcomprising: contacting an ammoniated solution with the process gas in anabsorption arrangement, the ammoniated solution capturing at least apart of the carbon dioxide of the process gas, wherein the molar ratio,R, of ammonia to carbon dioxide in the ammoniated solution is controlledsuch that substantially no precipitation of solids occurs within theabsorption arrangement; allowing ammoniated solution including capturedcarbon dioxide to exit the absorption arrangement; cooling theammoniated solution that has exited the absorption arrangement, whereinat least a part of the captured carbon dioxide is precipitated as solidsalt; separating at least a part of the precipitated salt from theammoniated solution; heating the ammoniated solution from which the atleast a part of the precipitated salt has been separated, such thatsubstantially no solids are present in the heated ammoniated solution;and allowing the heated ammoniated solution to re-enter the absorptionarrangement.
 2. The method of claim 1, wherein the ammoniated solutionthat exits the absorption arrangement has an R of between 1.8 and 2.5.3. The method of claim 1, wherein the ammoniated solution that exits theabsorption arrangement has a temperature of between 15° C. and 20° C. 4.The method of claim 1, wherein the ammoniated solution that re-entersthe absorption arrangement has an R of between 1.8 and 2.5.
 5. Themethod of claim 1, wherein at least a part of the ammoniated solutionthat re-enters the absorption arrangement has a temperature of between0° C. and 10° C.
 6. The method of claim 1, wherein the cooling isperformed to a temperature of between 0° C. and 7° C.
 7. The method ofclaim 1, wherein the controlling of the R of the ammoniated solution inthe absorption arrangement is at least partly accomplished byintroducing a controlled amount of ammoniated solution having an R ofbetween 2.2 and 5.0 to the absorption arrangement, apart from theammoniated solution that re-enters the absorption arrangement.
 8. Themethod of claim 1, wherein the separating is performed using ahydrocyclone.
 9. The method of claim 1, wherein the controlling of themolar ratio is automatic.
 10. The method of claim 1, wherein thecontrolling of the molar ratio uses a controller to maintain the molarratio in the absorption arrangement within a desired range.
 11. Themethod of claim 10, further comprising receiving a signal from a sensorproviding a signal indicative of the molar ratio within the absorptionarrangement.
 12. The method of claim 11, further comprising actuating afluid flow adjusting device in response to the signal received from thesensor.
 13. The method of claim 10, further comprising actuating a fluidflow adjusting device to control at least one of the ammonia and carbondioxide within the absorption arrangement.
 14. The method of claim 1,further comprising maintaining the NH₃-to-CO₂ mole ratio (R) of theammoniated solution in a first sump vessel of the absorption vesselwithin a range of 1.8 to 2.5, and to maintain the temperature of thefirst sump vessel within a range of 10 to 25° C.
 15. The method of claim1, further comprising maintaining the NH₃-to-CO₂ mole ratio (R) of theammoniated solution in a second sump vessel of the absorptionarrangement within a range of 2.0 to 4.5, and to maintain thetemperature of the second sump vessel within a range of 10 to 25° C. 16.The method of claim 14, further comprising introducing NH₃ or a mediumhaving an NH₃-to-CO₂ mole ratio (R) higher than the R of the ammoniatedsolution in a sump vessel into another sump vessel of the absorptionarrangement.